Paper handling systems frequently include folding subsystems, inserter subsystems and sorting subsystems in combination with other subsystems such as postage meters, and stackers, although each of these subsystems can be separate stand-alone systems.
Posts and private carriers frequently provide discounts to mailers who presort mail. The discounts vary from country to country and are often dependent on the level of presort. The more specifically the mail has been sorted in relation to delivery by the Post or carrier, the greater the discount. In the United States, mail sorting equipment reduces the costs of sorting mail, which may be from $45 per 1,000 pieces when manually sorted, to $3 per 1,000 pieces when automatically machine sorted. Furthermore, when mailers presort mail before presentation to the United States Postal Service (USPS), the postal discounts from the full postal charge for first class mail presently provided to the mailer by the USPS can be as high as 9.5 cents per mail piece. This discount may translate into savings for the mailer of as much as $3,800 per hour in postage alone if a sorter is processing 40,000 mail pieces per hour. The actual savings to the mailer may be greater since the capital cost of the equipment can be more than covered by the labor savings since such equipment can be operated with as few as two operators rather than the more than 60 which may be required to achieve such a rate manually.
These mail sortations implemented by the mailer, by the Posts or the private carriers often utilize a multiple pass radix sort algorithm. The United States National Institute of Standards and Technology defines a radix sort as a multiple pass distribution sort algorithm that distributes each item to a bucket according to part of the item's key beginning with the least significant part of the key. After each pass, items are collected from the buckets, keeping the items in order, then redistributed according to the next most significant part of the key. In a mailing system radix type sortation, the key can be the delivery address delivery code such as a ZIP code and the bucket can be the destination sortation bin. Use of a radix sort allows mail pieces to be sorted into delivery point sequence (carrier walk sequence) and eliminates the need for the delivery person to sort the mail before delivery. However, in implementing multipass sortations of this type, to achieve a delivery point sequence requires that the ordering of mail from prior sortations be maintained when the mail pieces from each of the sortation bins are combined for the next sortation pass.
There is great interest on the part of Posts, private carriers and mailers in improving the efficiency of the sortation process since it reduces the need to purchase additional equipment, to allocate space for the significant footprint associated with each machine, to merge multiple mail streams and to utilize additional equipment operators. Prior efforts to increase the sortation processing speed have involved advancing and separating mail in a sorter at ever increasing speed so that one piece is separated at a time from the stack at higher speed, transported to the sortation station at higher speed and moved or diverted into the sortation bins at higher speed. Such speed increases carry a high cost for implementation, narrow the range of mail that can be processed, and increase the damage to mail when jams occur.
Paper handling and mechanical considerations make improvements in sortation throughput difficult to achieve and technical improvements are typically very incremental. Typical throughput for mail piece sortation equipment has presently peaked at roughly between 30,000 and 40,000 pieces per hour, depending on the length of the mail pieces and their uniformity with peak transport speeds of about 180 inches per second (ips). Moreover, depending upon the particular equipment, at higher transport speeds, mail piece diverter gates may not reliably divert the mail pieces, the tracking logic may not reliably differentiate the mail pieces, the printers may not reliably and correctly print barcodes on the mail pieces, and the scanners may not reliably read the barcodes on mail pieces.
Typically, mail pieces are transported in sortation and other equipment along a path parallel to the mail piece long axis. The transport of the mail piece can be with the mail piece oriented on its edge or with the mail piece oriented on its side. The long mail piece axis transport helps to achieve good control of the mail piece while also allowing a gap in the belting or transport mechanism to permit viewing of the mail piece for scanning by an optical character reader (OCR) or a barcode reader (BCR). While manufacturers have experimented with transporting the mail pieces in a path perpendicular to their long axis to increase throughput, the variable lengths of mail pieces, the need to precisely position the printed information such as a USPS POSTNET barcode, and the difficulty of diverting and stacking mail complicate this approach.
In general, sortation throughput has been achieved through incremental increases in mail transport belt speed, although at higher speeds there may be a need for a greater gap for a given diverter response time and faster diverter gates. Improvements are thus incremental and also expensive. Prior USPS solutions have utilized several approaches in existing systems to overcome these problems. USPS accepts presorted trays of mail arranged by 5 digit destination (ZIP) code. This allows a single handling operation (at each decision point before the destination post office) for each tray containing hundreds of mail pieces rather than requiring that each mail piece be sorted and processed individually. Another USPS approach, such as the prototype Delivery Barcode Sorter (DBCS), employed multiple feeders feeding mail into one transport to allow manually and automatically processed mail to be merged. This approach does not increase automated mail transport throughput but rather allows full production speeds to continue while mail is being processed manually. The merging of two mail streams into a single sortation helps keep the machine running continuously since manually processed mail is very slowly produced. In yet another approach, the USPS flats sorting machine (FSM) model 775 systems with 4 feed stations at one end were modified (renamed FSM 881) to place 2 feed stations at each end of the machine. This allowed mail to be introduced at one end into transport carriers that had become empty because the mail had been out sorted at an earlier point in the transport. None of these solutions, however, are able to increase the transport capacity beyond a single envelope at a time passing down the transport path; they merely attempt to speed up existing processes and attempt to ensure that there is no unused capacity in the transport path.